I quite like your idea that the random encounters change because the PCs get a reputation for being too powerful to defeat, and thus only bigger and badder monsters are willing to take a chance. You can even add to this effect by having occasional random encounters that consist of weak opponents that used to trouble the PCs (orcs, lizardmen, etc.) coming to offer gifts or trying to swear fealty to the PCs . . .

Hehe, I thought about orcs and lizardmen attempting to talk to the PCs, just as the PCs would have tried to talk to the blue dragon they met back when they were 5th level, but offering gifts and swearing fealty? Priceless, haha. I must try that!